Project 1 will seek to identify genetic modifiers of Facioscapulohumeral muscular dystrophy by focusing on nonmanifesting earners of the 4qA allele. Several carriers of the 4q161PAS allele have been identified in Italy who have no manifestations nor family history of the disease. In addition, our Wellstone Center has identified families containing affected and nonmanifesting relatives who share the same allele and similarly express DUX4-FL in skeletal muscle. These individuals suggest that the current genetic signature of FSHD (D4Z4 contraction in the presence of the 4qA allele and a polyadenylation sequence in a distal pLAM sequence) is not an exclusive determinant of FSHD and that there is either a second-hit resulting in disease or a protective gene resulting in muscle health. Project 1 has two broad aims. The first aim is to identify and characterize nonmanifesting carriers of the FSHD allele. 10 nonmanifesting carriers without family history have been identified and examined in Italy and 4 nonmanifesting carriers from FSHD families have been identified and examined in the U.S. to date. Additional subjects will be recruited from the FSH Society and a large epidemiological study by the Italian National Registry for FSHD. Subjects will have detailed genotypic and phenotypic characterization including determination of DUX4-fl expression, muscle strength, muscle histopathology and muscle imaging. The second aim will identify genetic modifiers of FSHD through the use of whole-exome sequencing of nonmanifesting and affected individuals. In addition to an unbiased analysis of all -20,000 human genes, we will also test targeted hypotheses regarding whether variants in any of the ~300 known muscle-disease related genes are associated with FSHD manifestion, singly or collectively. Project 1 will interact closely with Project 2 where RNAseq will generate RNA sequence information that will confirm and add to the exome sequence data to identify genetic changes related to modifiers. Project 1 will also identify subjects that will be the source of donation of muscle and myoblasts for xenografts in Project 3. Finally, Project 1 will be assisted by the Cell Core which will provide histopathological analysis and CD56 myoblasts FACS sorted from muscle biopsy specimens.